A conventional fuel property detection device as disclosed in, for example, JP-A-5-133886, is configured to cause light emitted from a light-emitting element to be reflected off a boundary surface of a prism in contact with gasoline, thereby detecting the degree of heaviness of the gasoline based on a change in output from a light-receiving element by using a correlation between the degree of heaviness of the gasoline and a refractive index thereof. The fuel property detection device also detects an alcohol concentration contained in the gasoline using a correlation between the alcohol concentration in the gasoline and an electrostatic capacity between a pair of electrodes.
The above-described fuel property detection device can detect the degree of heaviness of the gasoline and the alcohol concentration in the gasoline. The fuel property detection device, however, includes a gasoline heaviness degree detector with the light-emitting element and the light-receiving element, and an alcohol concentration detector with a pair of electrode plates. The fuel property detection device further includes two systems of electric circuits for controlling detecting operations of the respective detectors. Thus, the size of the fuel property detection device becomes large, and the electric circuits for control of the fuel property detection device become complicated.